Ceramic tetrode



Unite 2,794,933 CERAMKC rnrinonn Application March is, 1952, Serial No. 276,740 2 Claims. c1. sis-n45 Our invention relates to an electron tube having a ceramic envelope and more particularly to such a tube embodying a tetrode structure.

It is among the objects of our invention to provide a tetrode having cylindrical electrodes incorporated in a coaxial ceramic envelope.

Another object is to provide such a tube in which a portion of the envelope is formed by ceramic sections and another portion is formed by a metal anode.

Still another object is to provide a tube wherein the ceramic envelope sections are metallically bonded together and in which the metallic bonds serve as lead-in conductors for the cathode, control grid and screen grid.

The invention possesses other objects and features of advantage, some of which with the foregoing, will be set forth in the following description of our invention. It is to be understood that we do not limit ourselves to this disclosure of species of our invention, as we may adopt variant embodiments thereof within the scope of the claims.

Referring to the drawings:

Figure 1 is a side elevational view of the tetrode embodying our invention; and

Figure 2 is a vertical sectional view of the same.

In terms of broad inclusion our improved tetrode comprises an envelope made up of vertically disposed ceramic sections, certain of the sections having outer cylindrical surfaces of different diameters decreasing in size toward the lower end of the envelope, which ceramic sections are metallically bonded together at the joints. A cup-shaped metal anode forms the upper portion of the envelope and is fastened to a ceramic section. Electrodes within the envelope, including cathode, control grid and screen grid, are of cylindrical structure and are coaxial with the ceramic sections. These electrodes are mounted on tubular supports connected to the metallic bonds at the joints, and the electrode terminals comprise metalized areas on the cylindrical surfaces of the envelope sections connected to the electrodes through the metallic bonds.

In greater detail and referring to the drawing, the tetrode embodying our improvements comprises an evacuated envelope, the lower portion of which is made up of vertically disposed coaxial sections of ceramic, namely, a lower stem section 2 and the cylindrical side wall sections 3, 4 and 6. Sections 2, 3 and 4 have outer cylindrical surfaces of different diameters decreasing in size toward the lower end of the envelope for the purpose of providing off-set coaxial terminals as hereinafter described.

To facilitate assembly the ceramic sections are prefer ably interfitted at the joints for self-alignment of the parts when the sections are stacked together. This is accomplished by recessing the sections to provide annular seats for receiving adjacent sections so that they are seated one against the other to align the parts coaxially and vertically.

The ceramic envelope sections in our tube are metallically bonded together at the joints to form vacuum-tight seals; the stem and wall sections 2 and 3 being united by Q metallic bond 7,.the wall sections 3 and 4 being united by metallic bond 8, and the wall sections 4 and 6 being united by metallic bond 9. These metallic bonding layers extend along the abutting surfaces between parts from the inside to the outside of the envelope. and also function as lead-in conductors for the electrodes as hereinafter described in greater detail.

The ceramic used in making up the envelope 'may be of'any suitable ceramic-like material, such as the alumina or zircon type bodies commercially available. We prefer the above ceramic bodies because their mechanical strength, thermal resistance and electrical insulating properties are favorable, although other type ceramics are also satisfactory.

Metallic bonds 7, 8 and 9 forming the vacuum-tight seals may be made in several ways, utilizing known metalizing and brazingtechniques. For example, the opposed surfaces of the ceramic pieces may be coated with finely divided molybdenum powder, or a mixture of molybdenum and iron powders or the like, and then fired in hydrogen to a temperature of about 1500 C. to sinter the metal powder to the ceramic surface. This produces a thin metallic layer firmly bonded to the ceramic. Such metalized surfaces may then be brazed or soldered together with silver solder or brazing alloys such as silver-copper, gold-copper or the like. The brazes are readily made by fitting the metalized ceramic pieces together with rings of wire solder adjacent the joints, and then elevating the temperature of the whole up to the melting point of the solder in a suitable furnace. Another metalizing technique is to paint tianium or zirconium hydride powders on the surfaces of the ceramic parts and fire in vacuum to about 1200 C., after which the metalized surfaces may be brazed together with a silver solder or the like. We prefer the molybdenum sintering process because it does not require a vacuum furnace for the firing operation.

In our tetrode the electrodes are of cylindrical construction and are coaxial with the axis of the tube. Anode 11 is of the external type and forms the upper portion of the envelope, the cup-shaped anode being of metal such as copper. This anode is fastened to the upper ceramic wall section 6, preferably by having its flange 12 brazed at 13 to the metalized edge of the ceramic section. A metal exhausttubulation 14 on the upper end of the anode is pinched off at 16 after evacuation of the envelope. The other electrodes including cathode 17, control grid 18 and screen grid 19 are located within the envelope concentric with the anode.

Cathode 17 is preferably of the indirectly heated type and comprises a cup-shaped cathode sleeve such as nickel enclosing a heater coil 21, the cylindrical surface of the cathode being coated with an electron emissive material such as the conventionl barium-strontium oxides. The cathode is carried by a tubular metal support 22 fitted at the lower end over a cylindrical upwardly projecting portion 23 of the ceramic stem section 2, which projection serves both as a fastening member and as means for aligning the cathode coaxially of the envelope. Projection 23 is formed as a reduced upper portion of the ceramic stem section and is arranged so as to place the lower end of support 22 adjacent bonding layer 7 at the lower joint. Support 22 is fastened in place and simultaneously connected to bonding layer 7 by brazing material 24 lying as a fillet around the base of the support and above the bonding layer at the joint. Braze 24 may be made at the same time that the braze is made at the joint 7.

As hereinbefore mentioned, the metallic bonds between the ceramic sections are utilized as lead-in con ductors to the electrodes. In the case of the cathode the bond 7 serves as the lead-in conductor. The external terminal 26 for the cathode is formed by metalizing the outer cylindrical surface of ceramic stem section 2. Thus,.when the ceramic section istreated, as by molybdenum sintering, to metalize the region .adjacent the joint, it is preferably metalized over substantially its entire length. so as to simultaneously provide the external terminal 26. l

Control grid 18 of our tetrode is of the wire cage type and is carried by a tubular metal support:27 having a base flange 28 fitted at the lower end over a cylindrical upwardly projecting portion 29 of theceramic wall section 3. This projection locates the lower, end of support adjacentthe intermediate joint at braze 8 and also serves to align the grid axially of the tube. The support is fastened in place and also connected to bonding layer 8 by brazing material 31 lying as a fillet around the support, which braze 31 may be made simultaneously with the braze at joint 8. Control grid terminal 30 is formed by metalizing the outer cylindrical surface of ceramic section 3,

which metalized area is formed as a continuation of that l at the joint 8. t

In a like manner the screen grid 19 is carried by a tubular metal support 32 having a base flange 33 fitted over a cylindrical projection 34 onceramic section 4, and is fastened by braze 36 connected to bonding layer 9 at the 'upper joint. Screen grid terminal 37 is formed by metalizing the area on the ceramic sections adjacent the joint 9.

The above described structure provides, a tetrode in which the cylindrical electrodes are mounted concentrically about the tube axis and in which the electrode terminals are coaxial with the electrodes. The stepped construction of the cathode, control grid and screen grid terminals is good for socket arrangements. Such a tube is well adapted for use in coaxial cavity circuits employed at ultra-high frequencies.

The brazes at the joints and metalized areas for the terminals are shown as having appreciable thickness for convenience of illustration. Actually, these are quite thin metallic layers, say of the order of 0.002" thickness, and appear as films or metal skins on the surfaces of the ceramic. If desired, silver, copper or the like may be flowed or electroplated over the sintered areas to further improve the electrical conductivity of such areas. Copper plating on the sintered areas, for example, makes excellent terminal surfaces and is ideal for brazing the surfaces together at the joints;

In the tube illustrated having an indirectly heated cathode, the cathode heater is preferably a coil 21 of small tungsten wire embedded in a layer of insulating material bonded to the inner surface of cathode sleeve 17. The ends of coil 21 are connected to metal heater leads 38 projecting upwardly from stem section 3, a heat'shield 39 being preferably fastened to one of these leads at the base of the cathode. Leads 38 preferably extend downwardly through holes in ceramic. section 2 to provide terminal prongs, the leads being sealed by brazes 41 bonded to metalized areas on the ceramic at the upper endsof the holes. While separate leads are shown for 4! the heater it is understood that one end of the heater may be connected to the cathode, in which. case only a single heater lead is required.

Our improved tetrode construction above described is particularly well suited for tubes of small size where compactness of structure is an important factor. The tube is extremely strong mechanically and has excellent thermal resistance properties for high temperature operation.

Still another important advantage of our tube is that it is easy to assemble and is well adapted for economical high speed production.

No additional cooling means need be provided on the anode if the tube is operated while immersed in a liquid such as oil. For other applications a suitable cooler, such as the conventional type of finned air cooler, may be provided on the anode.

We claim:

1. A tetrode having an envelope comprising a series of vertically disposed coaxial sections of ceramic joined together, metallic bonds uniting said sections at the joints, cylindrical electrodes in the envelope including a cathode and control grid and screen grid, a tubular metal support for the cathode connected to the metallic bond at a lower joint, a tubular metal support for the control grid connected to the metallic bond at an intermediate joint, a tubular metal support for the screen grid connected to the metallic bond at an upper joint, certain of the ceramic sections having cylindrical projections extending upwardly within the envelope and engaging the lower portions of respective tubular supports peripherally and in transverse planes, whereby axial alignment and vertical spacing of suchelectrodes is established, and an anode united with an upper ceramic section.

2. An electron tube having an envelope comprising a series of vertically disposed coaxial sections of ceramic joined. together, metallic bonds uniting said sections at the joints, cylindrical electrodes in the envelope, tubular metal supports for said electrodes, each support being connected to a ditferent one of said metallic bonds, certain of the ceramic sections having projections Within the envelope, each. projection having a surface defining a transverse horizontal plane and a cylindrical surface, certain of said electrode supports having corresponding surfaces for engagement with the horizontal and cylindrical surfaces on respective projections, whereby axial alignment and vertical spacing of the electrodes carried by such supports is established.

References Cited in the file of this patent UNITED STATES PATENTS 

